History may not endorse the Ulster secretary Shaun Woodward's reported claim that today's vote in the Northern Ireland assembly on the devolution of justice and policing is the most important in Northern Ireland history. Mr Woodward's hyperbole aside, it is immensely significant that the vote goes through, both for the substance of the issues and because of what turns on it politically for Northern Ireland. Certainly the people of Northern Ireland are in no doubt about the issue. In a poll yesterday, 72% of them (including 70% of unionists and 86% of nationalists) backed the devolved powers, while a remarkable 91% want to see Democratic Unionist first minister Peter Robinson and his Sinn Féin deputy Martin McGuinness working together for the community.

In spite of such support, the refusal of the Ulster Unionists to vote with the other parties today could be highly destabilising, even if the vote goes through. A UUP decision to treat devolved policing as a surrender to Sinn Féin cannot remotely be justified on the merits of the case. It would simply be an opportunist move to scare loyalist voters away from the DUP in the upcoming general election. Much more seriously, it could encourage dissidents on either side of the sectarian divide to put the new agreement to a lethal test. No one can know what strains some fresh killings or a bombing might place on the new agreement. It would be massively irresponsible to stir this pot and it is not blackmail, as the UUP claims, to warn against such a dangerous act.

The Conservative leader, David Cameron, also needs to face up to what is at stake here. Mr Cameron has put a lot of commitment into restoring the electoral pact between his party and the UUP. He has encouraged the Marquess of Salisbury's reactionary romantic attempt to bring the Tories and the two large unionist parties back under one political roof. As a result he now finds himself simultaneously the leader of a party which began and backs the Northern Ireland peace process and the partner of a party which seems determined to prevent the final cornerstone of that process from being put into place today. This is not a cost-free contradiction in terms, as the US government and bipartisan congressional groups have now made clear. American economic investment in Northern Ireland as well as security are at stake.

Last night the UUP walked out of a meeting with Mr McGuinness after three minutes. If that was not playing to the gallery, it is hard to know what is. Northern Ireland politics has known its fair share of posturing, but this is irresponsible by any yardstick. Mr Cameron needs to decide which side he is on: the peace process or those who want to derail it. He can't be on both.